Polarimetric Observations of 15 Active Galactic Nuclei at High Frequencies: Jet Kinematics from Bimonthly Monitoring with the Very Long Baseline Array

We present total and polarized intensity images of 15 active galactic nucleiobtained with the Very Long Baseline Array at 7 mm at 17 epochs from 1998 Marchto 2001 April. At some epochs the images are accompanied by nearly simultaneouspolarization measurements at 3 mm, 1.35/0.85 mm, and optical wavelengths. Herewe analyze the 7 mm images to define the properties of the jets of two radiogalaxies, five BL Lac objects, and eight quasars on angular scales $\gtrsim0.1$ milliarcseconds. We determine the apparent velocities of 106 features inthe jets; for many of the features we derive Doppler factors using a new methodbased on comparison of timescale of decline in flux density with thelight-travel time across the emitting region. This allows us to estimate theLorentz factors, intrinsic brightness temperatures, and viewing angles of 73superluminal knots, as well as the opening angle of the jet for each source. Weanalyze the derived physical parameters of the jets. In nine sources we detectstatistically meaningful deviations from ballistic motion, with the majority ofcomponents accelerating with distance from the core. In six sources we identifyjet features with characteristics of trailing shocks that form behind theprimary strong perturbations in jet simulations. The apparent speeds of thesecomponents increase with distance from the core suggesting an acceleration ofthe underlying jet.Comment: 100 pages, 44 figures, accepted for publication in AJ. This version does not contain figures 1-15,17(sequences of the images) due to very large size of the figures. The complete version (7mm_ms.ps) can be found at the website: www.bu.edu/blazars/multi.htm